Method for removing sulfur dioxide from the exhaust of a combustion furnace

ABSTRACT

THIS METHOD IS CHARACTERIZED BY THE FOLLOWING STEPS: CAUSING THE EXHAUST GAS OF A COMBUSTION FURNACE CONTAINING SO2 AND COKE OVEN GAS CONTAINING NH3 TO CONTACT TWO CIRCULATING SOLUTIONS SEPARATELY, IN EACH OF WHICH THERE ARE DISSOLVED AMMONIUM SULFITE, AMMONIUM BISULFITE AND AMMONIUM SULFATE; CIRCULATING A PORTION OF EACH OF SAID TWO SELF-CIRCULATING SOLUTIONS JOINTLY; CONTINOUSLY DRAWING OUT A PORTION OF SAID JOINTLY CIRCULATING SOLUTIONS; CONTINOUSLY SUPPLEMENTING A DECREASE OF SOLUTIONS IN ALL THE CIRCULATION SYSTEMS WITH FRESH WATER. AND MAINTAINING THE PH VALUE OF EACH CIRCULATING SOLUTION AT A LEVEL OF ABOUT 6.5 BY CONTROLLING THE FLOW RATES OF SAID COKE OVEN GAS AND JOINTLY CIRCULATING SOLUTION AND THE RATE AT WHICH THERE SHOULD BE DRAWN OUT A PORTION OF SAID JOINTLY CIRCULATING SOLUTION.

MINORU MIZUNO AL unmon FOR REMOVING SULFUR DIOXIDE mom THE EXHAUST OF A COMBUSTION FURNACE July 2, 1974 Filed May 23, 1972 $2; E zw o wxou modflwwm t w Q f u n i rill m 4L X25 s mmmmom Iz SE05 9 v N TX 52210 1|} mwmmom ew {J @9205 v w 0 v Q N United States Patent O flices US. Cl. 423--54 4 Claims ABSTRACT OF THE DISCLOSURE This method is characterized by the following steps: causing the exhaust gas of a combustion furnace containing S and coke oven gas containing NH to contact two circulating solutions separately, in each of which there are dissolved ammonium sulfite, ammonium bisulfite and ammonium sulfate; circulating a portion of each of said two self-circulating solutions jointly; continuously drawing out a portion of said jointly circulating solutions; continuously supplementing a decrease of solutions in all the circulation systems with fresh water; and maintaining the pH value of each circulating solution at a level of about 6.5 by controlling the flow rates of said coke oven gas and jointly circulating solution and the rate at which there should be drawn out a portion of said jointly circulating solution,

This invention relates to a method for removing injurious sulfur dioxide from the exhaust of a fuel combustion furnace or sintering apparatus, and more particularly to the one concurrently capable of removing ammonia from coke oven gas.

Recently, it has become an extremely important problem for prevention of air pollution to remove injurious sulfur dioxide from the exhaust of the boiler furnace of a steam power plant or the sintering apparatus of an ironworks. Although various purification methods are proposed in order to solve this problem, there has not yet been established any practical and economical method.

As a temporary attempt, expensive low sulfur fuel oil is often employed for a boiler. Along this line, there have been sintered raw ores mixed with iron ore or pyrite cinder containing as little sulfur as possible. However, such attempts are not fully effective to resolve the aforemen tioned serious problem of air pollution resulting from the uncontrolled release of sulfur dioxide, because they fail to carry out the complete desulfurization of exhaust gas and moreover require high cost.

Another known practice is dry desulfurization using expensive active carbon as an adsorbent. With this method, however, spent carbon should be regenerated for reuse by a very troublesome desorption process. Since, moreover, the sulfur dioxide thus recovered has to be changed into a harmless substance, this method requires a considerably complicated operation.

Still another known means is wet desulfurization using an alkaline liquor such as a water solution of caustic soda or a water suspension of slaked lime as an absorbent of sulfur dioxide. In this case, however, the spent absorbent can never be recovered, so that the operation of this method would be considerably uneconomical. Moreover, the waste liquor could not be put to any further practical use, and would have to be thrown away. This would, therefore, raise another problem of public pollution.

An object of the present invention is to provide an improved method for removing sulfur dioxide from the exhaust of a large scale fuel combustion furnace such as a 3,822,339 Patented July 2, 1974 boiler furnace of a steam power plant, with ease and at low cost.

Another object of the present invention is to provide a similar method suitable for desulfurization of the exhaust from an iron ore sintering apparatus of an ironworks.

A further object of the present invention is to provide a similar method concurrently capable of removing ammonia from coke oven gas which is present near the above-mentioned fuel combustion furnace or the sintering apparatus.

These objects can be accomplished in accordance with the present invention by the following steps:

(a) Continuously bringing a exhaust containing sulfur dioxide into contact with a first circulating water solution consisting of a mixture of ammonium sulfite, ammonium bisulfite and ammonium sulfate, the pH value of said solution being from 5.8 to 6.5, thereby dissolving said sulfur dioxide in said first circulating solution;

(b) Continuously bringing coke oven gas containing ammonia into contact with a second circulating water so lution consisting of a mixture of ammonium sulfite, ammonium bisulfite and ammonium sulfate, the pH value of said solution being from 6.3 to 7.0, thereby dissolving said ammonia in said second circulating solution;

(c) Continuously pouring a portion of said first circulating solution of step (a) into said second circulating solution of step (b) and concurrently pouring almost the same amount of said second circulating solution of step (b) into that of step (a), thereby forming a jointly circulation system;

((1) Continuously drawing off a portion of the jointly circulating solutions of step (c), and concurrently supplementing a decrease of the solutions in all the circulating systems with fresh water, and utilizing the drawn soluties to recover ammonium sulfate by means of oxidation; an

(e) Maintaining the pH values of the circulating solutions of steps (a) and (b) at prescribed level by controlling the flow rates of the coke oven gas of step (b) and the jointly circulating solution of step (c) and the rate at which there should be drawn off part of the jointly circulating solution of step (d).

Other important objects and advantageous features of the invention will be apparent from the following description and an accompanying drawing, wherein a specific embodiment of the invention is set forth in detail.

The appended drawing is a fundamental flow sheet of the process according to this invention.

The principle of the process in accordance with this invention consists in neutralizing the sulfur dioxide contained in an exhaust of the boiler furnace of a steam power plant or a sintering apparatus of an ironworks indirectly with ammonia contained in coke oven gas by a wet process.

In the figure, an absorber 2 is supplied with an aqueous dissolving liquor 1 containing ammonium sulfite, ammonium bisulfite and ammonium sulfate and having a pH value of from 5.8 to 6.5, and a total concentration of said ammonium salts is from 30 to 40% by weight. Exhaust gas from a combustion furnace 3 is passed through the absorber 2 by a blower 4 to contact the liquor 1 in said absorber, thereby dissolving the sulfur dioxide contained in the exhaust in said aqueous solution almost thoroughly. The desulfurized gas is released into the air through a chimney 18. In this case the exhaust gas is cooled in advance to a temperature of about 50 C. The liquor 1 is circulatingly introduced into the absorber 2 through a pump 5 and a liquor storage tank 6. In this system, the pH value of the circulating liquor decreases gradually as the sulfur dioxide is gradually dissolved therein unless another high pH solution is added.

Another absorber 8 is supplied with another aqueous dissolving liquor 7 containing the same salts as in the aforementioned liquor 1, having a pH value of from 6.3 to 7.0 and the same range of total concentration as in the liquor 1.

Exhaust gas from a coke oven 13 is passed through the absorber 8 by a blower 15 to contact the liquor 7 in said absorber, thereby disolving the ammonia contained in the coke oven gas in said aqueous solution almost thoroughly. The coke oven gas 17 thus treated to be almost stripped of ammonia is conducted to the place where it is utilized. The coke oven gas is also cooled in advance to a temperature of about 50 C. The liquor 7 is brought circulatingly into the absorber 8 through a pump 9 and a liquor storage tank 10. In this system, the pH value of the circulating liquor increases gradually as the ammonia is dissolved therein by degrees, unless another low pH solution is added.

In order to maintain the aforementioned pH values of dissolving liquors 1 and 7, a portion of the first circulating solution 1 is continuously poured into the second circulating solution 7 using a circulating pump 12, and almost the same amount of the second circulating solution 7 is also continuously poured into the first circulating solution 1 concurrently using another circulating pump 11, thus forming a jointly circulating solution.

Furthermore, it is required that a portion of the second circulating solution 7 be continuously drawn off from the storage tank to avoid the accumulation of ammonium salts in all of the circulating systems, and a suitable amount of fresh water 16 be concurrently poured into the storage tank 10 in order to supplement therewith a decreased amount of circulating solutions in all the circulating systems. The quantity of the circulating solutions also decreases due to evaporation of the liquors in the absorbers as well as the above-mentioned withdrawal of the second circulating solution 7. The drawn liquor is accumulated in another storage tank 14, and supplied to an ammonium sulfate recovering plant. The ammonium sulfite and ammonium bisulfite in the drawn liquor may be changed into ammonium sulfate by means of a well-known oxidation method without difficulty.

Most of the hydrogen sulfide in the coke oven gas dissolves in the second circulating solution 7 in the absorber 8. The hydrogen sulfide reacts with the ammonium bisulfite in the second circulating solution and the ammonia in the coke oven gas, and changes to ammonium thiosulfate, the chemical reaction being as follows.

Therefore, it is possible to remove a great deal of the hydrogen sulfide together with ammonia from the coke oven gas. The dissolved ammonium thiosulfate circulates through all the circulating systems, and is partly drawn off from the systems together with other ammonium salts, and further, is changed into ammonium sulfate by oxidation in the aforementioned ammonium sulfate recovering plant.

The main chemical reactions in the absorbers 2 and 8 are as follows:

As mentioned before, the pH values of the first and second circulating solutions 1 and 7 should be kept constant in the range of from 5.8 to 6.5 and from 6.3 to 7.0 respectively. The maintenance of these pH values is effected by controlling the flow rates of coke oven gas and the jointly circulating solution and the rate at which there should be drawn off a portion of said jointly circulating solution. As reactions in the absorbers 2 and 8 proceed very rapidly, the total concentrations of ammonium salts in all the circulating systems can be elevated to a relatively high level, for example, 30 to 40 percent, and

4 such high concentration is convenient to recover ammonium sulfate from the drawn solution.

Under the aforementioned conditions almost all of the sulfur dioxide contained in the exhaust and the ammonia contained in the coke oven gas can be removed easily. As a specific case the pH value throughout all the circulation systems may be set at a single level of about 6.4 (or 6.3 to 6.5). This arrangement, enables sulfur dioxide and ammonium to be eliminated from the exhaust of the combustion furnace and coke oven gas in as high amounts as more than respectively.

Ammonia gas contained in coke oven gas is generally removed by sulfuric acid. In the present invention, however, there is no need of using expensive sulfuric acid, rendering the whole operation very economical and harmless. Furthermore, the waste liquor from the ammonium sulfate recovering plant does not give rise to any public pollution.

According to this invention, hydrogen sulfide can be removed from coke oven gas in the absorber 8 to an extent of from 20 to 25 percent on the basis of the total weight of the hydrogen sulfide when the second circulating solution 7 has a pH value of from 6.3 to 6.5, and to a much higher extent of from 40 to 45 percent when said pH value ranges from 6.8 to 7.0.

As the pH value of the first circulating solution 1 is maintained at a relatively low level as 5.8 to 6.5, the absorber 2 can be safely operated at a temperature of about 50 C. without evolving the mist of ammonium sulfite. Moreover, this temperature enables outlet gas from the absorber 2 to be easily drawn off through the chimney 18 by its own buoyancy.

It is preferred that, during a normal operation, the total concentration of ammonium salts in all the circulating solutions be kept at from 30 to 40 percent as mentioned 'before. At the initial operation of a new plant, it is able to circulate water first throughout the circulation system. In this case, arrangement is made to cause the total concentration of ammonium salts to increase gradually during operation up to a prescribed level.

The absorber may consist of any of a spray tower, packed tower, multi-tray tower and wet wall tower, or if necessary several towers arranged in series or parallel. Further, where desired, either counter current system or parallel current system may be adopted for gas absorption.

The method according to the present invention may be favourably practised in the place where a fuel-fired furnace such as a boiler furnace and a coke oven are located near to each other.

The process of the present invention will be more fully understod by reference to the following examples.

EXAMPLE 1 Composition of exhaust gas (by volume):

CO percent 2.0 ,0 do 11.0 C0 do 0.5 $0 p.p.m 700 N Remainder Dust g./Nm. 0.3

Composition of coke-oven gas (by volume):

CO perc nt 2.0 C,,H do 4.0 CH, do 29.0 C0 do-.. 6.0

0 Trace H percent 56.0 N Remainder NH g./Nm. 8.4 H 8 g/Nm. 4.5

The exhaust gas cooled to about 50 C. was introduced into a SO -absorbing tower at a bottom thereof at the rate of 484 Nm. per hour, and an absorbing solution, having a temperature of 46 C. and a pH value of 6.3 was circulated through said tower at a rate of 35 liters per minute. The S0 content in the outlet gas from the tower was 22 p.p.m., showing that the gas was absorbed to an extent of 96.7%.

On the other hand, the coke-oven gas cooled to about 50 C. was introduced into an NH -absorbing tower at the bottom thereof at the rate of 177 Nm. per hour, and an absorbing solution having a temperature of 40 C. and pH value of 6.45 was circulated through said tower at the rate of 50 liters per minute. The contents of NH, and H 8 in the outlet gas from the tower were 0.88 g./Nm. and 2.7 g./Nm. respectively, indicating that NH and H 8 were absorbed to an extent of 87.3% and 45% respectively.

In order to maintain the above-mentioned rates of absorption, a portion of the circulating solution for S0 absorption and a portion of the circulating solution for NH absorption were made to be circulated jointly at the rate of liters per minute, and a portion of said jointly circulating solution was drawn off at a rate of 65 ml. per minute, and further water was supplied to said jointly circulating solution at the rate of 87 ml. per minute. The concentrations of ammonium salts in the jointly circulating solution were as follows:

EXAMPLE 2 After being cooled to about 50 C., exhaust gas from a sintering apparatus containing 1165 p.p.m. of S0 was introduced into the same SO -absorbing tower as in Example l at a rate of 438 Nm. per hour, and an absorbing solution, having a temperature of 48 C. and a pH value of 6.11, was circulated through said tower at a rate of 35 liters per minute. The S0 content in the outlet gas from the tower was 28 p.p.m., the S0 being removed as much as 97.7%.

On the other hand, a coke oven gas containing 5.1 g./Nm. of NH, and 5.1 gJNmS of H 8 was introduced, after cooled to about 50 C., into the same NH -absorbing tower as in Example 1 at a rate of 141 Nm. per hour, and an absorbing solution having a temperature of 45 C. and a pH value of 6.5 was circulated through said tower at a rate of 50 liters per minute. The contents of NH and H S in the outlet gas from the tower were 0.19 g./ Nm. and 3.8 g./Nm. respectively, the removal of NH and H 8 amounting to 96.6% and 25% respectively.

In order to maintain the above-mentioned rates of absorption, a portion of the circulating solution for S0 absorption and a portion of the circualting solution for NH;, absorption were made to be circulated jointly at the rate of 1.0 liter per minute, and a portion of said jointly circulating solution was drawn off at a rate of 65 ml. per minute, and further water was supplied to said jointly circulating solution at a rate of 87 ml. per minute. The concentrations of ammonium salts in the jointly circulating solution were as follows.

(NH.,) SO g./l 113.7 (NH )HS0 g./l 45.5 (NH4)2SO4 g./l '(NH S O g./l.... 55.7 Specific gravity of the solution 1.191 Concentration of total salts weight percent 32.0

What we claim is:

1. A method for removing sulfur dioxide from the exhaust of a combustion furnace comprising:

(a) continuously bringing said exhaust containing sulfur dioxide into contact with a first circulating water solution consisting of a mixture of ammonium sulfite, ammonium bisulfite and ammonium sulfate, the pH value of said solution being from 5.8 to 6.5, thereby dissolving said sulfur dioxide in said first circulating solution;

(b) continuously bringing coke oven gas containing ammonia into contact with a second circulating water solution consisting of a mixture of ammonium sulfite, ammonium bisulfite and ammonium sulfate, the pH value of said solution being from 6.3 to 7.0, thereby dissolving said ammonia in said second solution;

(c)continuously pouring a portion of said first circulating solution of step (a) into said second circulating solution of step (b) and concurrently pouring almost the same amount of said second circulating solution of step (b) into that of step (a), thereby forming a jointly circulating solution;

(d) continuously drawing off a portion of the jointly circulating solution of step (c), and concurrently supplementing a decrease of the solutions in all the circulating systems with fresh water, and utilizing the drawn solution to recover ammonium sulfate therefrom by means of oxidation; and

(e) keeping the pH values of the circulating solutions of steps (a) and (b) at prescribed levels by controlling the flow rates of coke oven gas of step (b) and the jointly circuating solution of step (c) and the rate at which there should be drawn 01f part of the jointly circulating solution of step (d).

2. A method according to Claim 1 wherein the pH values of all the circulating solutions are maintained at a level ranging from 6.3 to 6.5.

3. A method according to Claim 1 wherein the concentration of total ammonium salts in each circulating solution is maintained at 30 to 40 percent.

4. A method according to Claim 1 wherein the combustion furnace includes a boiler furnace and a sintering apparatus used for fine iron ores.

References Cited UNITED STATES PATENTS 1,740,342 12/ 1929 Hansen 423-242 2,134,481 10/ 1938 Johnston 423-242 1,931,408 10/ 1933 Hodsman et al. 423-242 1,043,210 11/1912 Doherty 423512 OSCAR R. VERTIZ, Primary Examiner G. HELLER, Assistant Examiner US. Cl. X.R. 423-238, 242 

